Sexual behavior is a crucial component of most adult relationships and has demographic, epidemiological and social psychological consequences. Unfortunately, the importance of the study of sexual behavior has not been matched by an increase in its understanding, especially the sexual behavior of adults in the U.S. Relatively little research has been based on couples, where reports are obtained from both partners. It is only from a couples-based study that we can gain a comprehensive, more accurate understanding of factors that affect the sexual decision making and behaviors of the couple, and how sexual decision making and sexual practices are related to relationship satisfaction and fidelity. The overall objective of the proposed study is to adopt a couples perspective and use a unique large-scale population based data set of approximately 950 adult heterosexual couples of all relationship types (married, cohabiting, and sexually active dating couples) in the U.S. to better understand adult sexual decision making and behaviors. In particular, we will address such central research issues as: 1) how personal and relationship characteristics are related to sexual decision making and sexual behavior; 2) how sexual decision making and behavior are related to relationship satisfaction and fidelity; 3) whether partner characteristics and reports improve our understanding of sexual decision making, sexual behavior, and relationship satisfaction and fidelity; 4) how power differences between the partners affect sexual decision making and the choice of sexual practices in which to engage; 5) how the effects of personal, relationship and partner characteristics on sexual decision making, sexual behavior, and relationship satisfaction and fidelity differ by gender and relationship type; 6) whether researchers can be confident in their conclusions about the effects of personal and relationship characteristics if they have data reported only by males or only by females; and 7) how personal, partner and relationship characteristics are related to the concordance of partner reports of sexual decision making, sexual behavior, and relationship satisfaction and fidelity. These key features of the proposed study argue that its successful completion will provide a major contribution to our understanding of adult sexual behavior in the U.S. The overall objective of the proposed study is to better understand adult sexual decision making and behaviors in the U.S. by adopting a couples perspective and using a unique large-scale population based data set of approximately 950 adult heterosexual couples of all relationship types (married, cohabiting, and sexually active dating couples). Sexual behavior is a crucial component of most adult relationships and has demographic, epidemiological and social psychological consequences. A comprehensive understanding of adult sexual behavior is therefore relevant to NICHD's mission of funding research that impacts the health and well being of individuals. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]